Employing the candidate genetic approach, we previously identified mutations in the gene encoding NEMO (NF- kB essential modulator), an intracellular signaling constituent of the NF-kB pathway, results in ectodermal dysplasia with an immune deficiency. Mutations in the zinc finger domain of NEMO block CD40 mediated activation of NF- kB and prevent B cells from undergoing class switch recombination (CSR) and APCs from synthesizing NF -kB regulated cytokines such as IL -12 or TNF-a when stimulated with CD40 ligand. The zinc finger domain of NEMO is a site for the covalent attachment of ubiquitin and this step is necessary for the full activation of NF-&#954;B. The majority of patients with ectodermal dysplasia with immune deficiency have mutations in the zinc finger domain and we have found that such mutations impair the post- translational modification of NEMO by ubiquitin. Related efforts in the laboratory include making genetically altered mice with targeted mutations in signaling molecules, which regulate NF-&#954;B. CYLD is a deubiquitinating enzyme that targets signaling constituents of the NF-kB signaling pathway, including NEMO. Alteration in CYLD have been described in patients with familial cylindromatosis, a condition characterized by numerous benign adnexal tumors. In mice deficient deficient in CYLD we previously showed that the development of B cells, T cells, and myeloid cells is unaffected in CYLD deficient mice, but that the activation of these cells with mediators of innate and adaptive immunity results in enhanced NF-kB activity and is associated with increased NEMO ubiquitination. CYLD deficient mice are more susceptible to induced colonic inflammation and show a dramatic increase in the incidence of tumors in a colitis associated cancer (CAC) model. These results suggest that CYLD limits inflammation and tumorigenesis by regulating NEMO ubiquitination in vivo. We are now studying the regulatory role of CYLD in other signaling pathways and are making a CYLD knockout NEMO knockin mice.